The present invention relates to a novel copolymer and the preparation thereof, and more particularly to a copolymer suitable for use in contact lenses.
The present invention also relates to contact lenses which can be worn continuously for a long term without pain, and more particularly to contact lenses made of a novel copolymer having an excellent oxygen permeability which is increased by introducing a siloxane at branching chain ends of a polymer chain.
Contact lenses are generally classified into two large groups of the hard type and the soft type.
The hard type contact lenses are made of polymethyl methacrylate. Such contact lenses have a history of several tens of years, but are still insufficient for use in visual correction. The contact lenses made of polymethyl methacrylate give a foreign body sensation to many persons and the use of the contact lenses is often given up in the early stage of wear. Also, even persons who can bear the foreign body sensation take about one month for accommodation before they can wear the contact lenses for over 12 hours, and for this trouble, some of them give up the use of the contact lenses. Further, even persons who have accommodated to the use of the contact lenses and have been in no pain, always have a feeling of wearing contact lenses on eyes. Also, when the wear of the contact lenses is stopped for several days, the period of accommodation is required again. That is to say, for wearing the contact lenses made of polymethyl methacrylate all day there are required a period of bearing a foreign body sensation and a period of accommodating to the foreign body. This is due to the fact that since the use of the contact lenses made of polymethyl methacrylate is a burden on a respiration of the cornea (which takes oxygen from air because of the nonvessel tissue) because of its low oxygen permeability, it is required to minimize interception between cornea and air by the contact lenses and, therefore, the size of the contact lenses must be made as small as possible and also the contact lenses must be worn to move on cornea by making the curve of the inner surface of the lens larger than the radius of curvature of cornea so that tears containing oxygen might readily circulate between cornea and the lens, by which users have a stronger foreign body sensation.
Also, there exist persons who cannot wear the contact lenses all day and only wear for 5 to 6 hours a day, since when wearing for a long time, ocular congestion occurs, vision becomes blur or they feel a weariness. This is caused by the fact that in case of such persons, since the amount of tears is small, the supply of oxygen is small and, therefore, cornea lacks in oxygen and transforms.
Accordingly, there has been desired contact lenses which does not give a foreign body sensation and can be comfortably worn without pain. Soft type contact lenses were developed to satisfy such demands. However, so developed soft contact lenses are also not satisfactory for the following reasons.
Two kinds of soft contact lenses are known. One is those prepared from poly-2-hydroxyethyl methacrylate, and the other is those prepared from a hydrophobic silicone rubber.
Since the silicone rubber lenses are very water-repellent and are greatly different from cornea in thermal properties such as thermal conductivity and thermal diffusivity, they give a foreign body sensation, particularly a burning sensation, despite that the oxygen permeability is very large and, therefore, there is required a stronger patience than the case of polymethyl methacrylate lenses to accommodate to the silicone rubber lenses. Further, the silicone rubber is soft and elastic, and precise mechanical treatments thereof such as cutting, grinding and polishing are very difficult. Also, many attempts to make the surface of silicone rubber lenses hydrophilic have been reported, but a satisfactory silicone rubber contact lens has never been developed.
The contact lenses made of poly-2-hydroxyethyl methacrylate absorb water and become flexible and, therefore, the stability of visual power is designed by making the size of lens large. Accordingly, a feeling in wear is very good. However, since the water content of the lens is at most 40 % by weight, the permeation of oxygen through water as a medium is small and it is impossible to continuously wear the lenses for a long period of time. According to Journal of Japan Contact Lens Society, Vol, 12, No. 10, 142 (1970), it is reported that oxygen required for the cornea in respiration is about 12 to 20 mmHg. in partial pressure of oxygen on cornea during wear of contact lenses. In case of the contact lenses made of poly-2-hydroxyethyl methacrylate having a thickness of about 0.2 mm., the oxygen partial pressure is only about 5.5 to 6 mmHg. Therefore, the problem of corneal respiration cannot be completely solved also by such contact lenses. For such a reason, the contact lenses made of poly-2-hydroxyethyl methacrylate are worn flat so as to move with every winking, by which tears can circulate between cornea and lens. Therefore, the lens are gradually soiled during wear or by awkward handling and when the movement of the lens on cornea becomes small, exchange of tears decreases and in conjunction with the lens being large in size, corneal troubles become easy to occur.
As stated above, it is an essential condition for enabling the continuous, safety wear of contact lenses for a long term that the oxygen permeability is large, and it is desirable for comfortably wearing contact lenses that a foreign body sensation is as small as possible.
From such a point of view, high water content contact lenses having excellent oxygen permeability which are made of a polyvinyl pyrrolidone as a main component have been proposed and reported, but these contact lenses are also unsatisfactory for continuous wear. For instance, polyvinyl pyrrolidone lens of a certain kind is extremely colored or is translucent. Also, a certain lens is so weak in quality of material that a user himself cannot handle it freely. In general, a material of high water content has the defects that the stability in shape of lens is bad as a result of containing water in large quantities and, therefore, the dimensions of lens may easily be changed even by changes in temperature of tears, pH and osmotic pressure, and that a lens contour transforms during the use of a long term and the refractive power, size and base-curve of the lens changes, by which the life of the contact lens is shortened. Further, there is a serious problem in the lens of this type that the lens with high water content is contaminated by bacteria. For solving this problem, boiling treatment, chemical sterilization treatment, etc. are considered, but these are troublesome. Moreover, the boiling treatment accelerates the deterioration in quality of material and the chemical sterilization treatment incurs danger to eyes.
Further, the high water content contact lens has the fatal disadvantage that the optical properties change at the time of wearing. That is to say, the water content of the contact lens undergoes a change with external environment, and this brings about changes in refractive index, size, base-curve, front-curve and transparency of the lens, by which the visual correction ability is lowered. Such a change in water content of the lens is mainly caused by the evaporation of water from the surface, particularly the outer surface of the lens. The higher the water content and the thinner the lens, the larger the change in optical properties. At times, the evaporation rate of water is too large and the water supply to a lens by tears is late, and as a result, the edge of the lens is warped and sometimes the lenses fall from eyes.
As stated above, conventional contact lenses are not necessarily suitable for continuous wear for a long term.